Here's the Fall newsletter of Topics in Subtropics, and it is on time. Winter hasn't started yet, but get ready.
And the topics are:
- Real-Time Sensor for Early Detection of Citrus Huanglongbing (HLB)
- The 2016 International Citrus Conference
- UC Riverside Scientists Evaluate Trunk Injections of Pesticides for The Management of Ambrosia Beetles in California Avocados
- An Overview of Mango
- Water Based Latex Paint as a Means to Track Ambrosia Beetle Activity on Infested Trees
The drought has caused numerous conditions – physiological and pathological – that I have only seen in text books (see our newsletter article: http://ceventura.ucanr.edu/newsletters/Topics_in_Subtropics63007.pdf). But other phenomena are also occurring. Recently I saw a field of blackberries in full bloom and the other day a grower called in about a plum tree that was also in full bloom. What is going on? This is supposed to happen late winter/early spring. It turns out that often drought stress can supplant winter chill in some plant species. In this case, these two species are relatively low chill, meaning they don't require a lot of cold to break winter bud dormancy. The drought stress causes the buds to break dormancy.
This is similar to the “Verdelli” effect in lemons. This is a technique used to shift the period of optimum fruit production to a more profitable period, usually the summer when more lemons are used. In the case of plum and blackberry and other low chill deciduous tree crops, this would be pushing production into the coldest period of the year. It might work along the coast, but in the Central Valley it would probably just mean frozen fruit. But it's a possible method that we might play with.
Photo: October, 2016
The roof rat (Rattus rattus), sometimes called the black rat, is a common vertebrate pest in citrus, avocado and other yummy tree orchards. It builds leaf and twig nests in fruit trees or nearby trees, or it can nest in debris piles or thick mulch on the ground. This agile, sleek rat has a pointed muzzle, and a tail that is longer than the body and head combined.
Be sure to identify the species of rat present to avoid killing nontarget or protected species. Be aware that endangered native kangaroo rats (Dipodomys spp.) and the riparian woodrat (Neotoma fuscipes riparia) resemble pest rats, but are protected by law. Unlike the hairless, scale-covered tail of roof rats, the tails of kangaroo rats and the riparian woodrat are covered with fur. The riparian woodrat is active mostly during the day, and its tail is somewhat shorter than the combined length of its body and head. A kangaroo rat's tail is noticeably longer than its body and head combined. Kangaroo rats are nocturnal, but unlike Norway rats and roof rats, which move on all four legs, kangaroo rats hold their front legs off the ground and travel by hopping on their hind legs.
Rats gnaw on electrical wires, wooden structures, and fruit on trees. After harvest, they damage fruit in bins, chewing on the bins and leaving excrement. Rats are active throughout the year, and mostly at night.
To help manage rats, reduce shelter and nesting sites of rats. Eliminate debris and wood piles. Store materials neatly and off the ground. Thin and separate non-crop vegetation around orchards where feasible. Exclude rats from nearby structures by properly sealing entry ways.
Baits and rat-sized snap traps placed in trees are the most effective control measures. Rats are wary, tending to avoid baits and traps for at least a few days after their initial placement. Fasten traps to limbs and bait them with sweet fruit or nut meats, but do not set the traps until after bait is readily eaten. Secure anticoagulant wax blocks in a bait station before placing in trees on limbs 6 feet or more above the ground. Placing the wax blocks in a bait station will prevent chunks of the anticoagulant wax from dropping to the ground and creating a hazard.
Be aware that certain types of single-dose rat baits for use inside buildings are not labeled for use outdoors in orchards; these are hazardous to wildlife and should not be used.
For more on the subject see:
and another blog:
The talk of drought and water restrictions in the State has created a time for serious decisions. What can be done with avocado citrus trees that have been invested with time and money when there are allocations of water? Although this article is addressed to subtropicals specifically, the guidelines are generally applicable to all fruit trees.
Irrigation systems and scheduling
One of the surest, although not necessarily the cheapest, ways of managing with a decreased amount of water is to improve its application and scheduling. Insure that equipment is working properly, that nozzles are not plugged or worn, that pressures in the irrigation blocks are uniform, that leaks are repaired, and that no runoff or deep percolation are occurring. Many Resource Conservation Districts have Mobile Labs that can help make a system evaluation.
In the Ventura area, oranges use about 30 inches of water per year, however , the monthly amount varies with weather. Applying water in the amounts and at times for optimum production can be improved by using tensiometers. A gauge reading near 40 centibars has been recommended for the one foot tensiometer. The three foot tensiometer can be used to determine the amount of moisture stored in the lower horizon and to determine whether the irrigation was effective, whether the irrigation water infiltrated down to that depth.
Whatever reading is used there is no substitute for observation of the trees themselves and the soil.. Use a soil sampler or shovel to verify the depth of water applied. If time clocks are being used, turn them off or at least adjust them frequently enough to accommodate changing weather patterns. Use of CIMIS weather data can aid in correcting schedules to changing weather.
If water applications need to be curtailed, there will be a decline in yield and fruit size. Applying something less than about 75% of tree requirement will give reduced yields not only for this year, but will lead to dieback and low yields for 3 -4 years after. Abandoning the trees altogether will also yield little or no crop and dieback, but the trees will often return to normal yields in 3 - 5 years. If little water is available, it may be best for commercial operators to reduce the number of trees irrigated to those that can receive 85% of their water requirement and abandon the rest, hoping for more water in future years.
Since it is the leaves that are the site of water loss , the best way to reduce water loss is to reduce the amount of leaves present. This is the ideal time to thin an orchard, get rid of those trees that are shading each other and reducing the per tree yield of fruit. This is a good time to topwork trees to better varieties, since the smaller trees will use less water. A good weed management program will reduce competition for water, and mulching the wetted area of the sprinkler will reduce evaporative loss from the soil surface. Once the leaf area is reduced, it is necessary to adjust the irrigations to reflect the decreased need for water.
This is an opportunity to identify the least productive trees in an orchard and cut of water to them. Trees with root rot or frost damage; trees growing on limy/iron chlorosis sites. Trees growing on ridges that receive the full force of the wind and have a lower yield per gallon of water should be considered first. Trees growing on the perimeter of an orchard also will transpire more water for a given amount of fruit. If all trees in the orchard look good, then these perimeter trees should be targets for saving water. If production records have been kept for different blocks of trees, it might be possible to identify low yielding areas that could be sacrificed.
This is an opportunity , as well. Many growers have kept poor producing parts of groves going because it is an emotional issue to cut up a tree. Seize the day and take advantage of the situation.
For more, check out the powerpoint
- Author: Gary Bender
- Author: David Shaw
OK! Let's Strategize. There are four steps for everybody to consider, it doesn't matter if you have a backyard lawn and landscape or if you have 700 acres of avocados.
1. Maintenance: Irrigation System and Cultural Practices
2. Improve Irrigation Scheduling
3. Deficit Irrigation
4. Reduce Irrigated Area
a. Irrigation System.
- Fix leaks. Unfortunately, there are almost always leaks for all kinds of reasons. Pickers step on sprinklers, squirrels eat through polytube, branches drop on valves, coyote puppies like to chew….the system should be checked during every irrigation
- Drain the lines. At the beginning of each year every lateral line should be opened in order to drain the fine silt that builds up.
- Maintain or increase the uniformity of irrigation so that each tree or each area gets about the same amount of water. Common problems include different sized sprinklers on the same line or pressure differences in the lines. Where there are elevation changes, every line should have a pressure regulator, they come pre-set to 30 psi. Having all of your lines set up with pressure regulators is the only way you can get an even distribution of water to all of the trees, and it solves the problem of too much pressure at the bottom of the grove and not enough at the top.
- Clean the filters often. You don't have a filter because you think that the district water has already been filtered? Hah! What happens if there is a break in the line in the street and the line fills with dirt during the repairs? All of your sprinklers will soon be filled with dirt.
- Is water flow being reduced at the end of the lateral line? It could be because scaffold roots are growing old enough to pinch off the buried line. The only cure is to replace the line.
b. Cultural Management.
- Control the weeds because weeds can use a lot of water.
- Mulch? Mulching is good for increasing biological activity in the soil and reducing stress on the trees, but the mulch will not save a lot of water if you are irrigating often….the large evaporative surface in mulches causes a lot of water to evaporate if the mulch surface is kept wet through frequent irrigation. Mulches are more helpful in reducing water use if the trees are young and a lot of soil is exposed to direct sunlight.
2. Improve the Irrigation Scheduling.
- CIMIS will calculate the amount of water to apply in your grove based on last week’s water evapotranspiration (ET). You can get to CIMIS by using several methods; for avocado growers the best method is to use the irrigation calculator on the www.avocado.org website. If you need further instruction on this, you can call our office and ask for the Avocado Irrigation Calculator Step by Step paper. You need to know the application rater of your mini-sprinklers and the distribution uniformity of your grove’s irrigation system.
- CIMIS tells you how much water to apply, but you need tensiometers, soil probes or shovels to tell you when to water.
- “Smart Controllers” have been used successfully in landscape and we have used one very successfully in an avocado irrigation trial The one we used allowed us to enter the crop coefficient for avocado into the device, and daily ET information would come in via a cell phone connection. When the required ET (multiplied automatically by the crop coefficient) reached the critical level, the irrigation system would come on, and then shut down when the required amount had been applied. Increased precision can be obtained by fine tuning these devices with the irrigation system precipitation (application) rate.
3. Deficit Irrigation.
- Deficit irrigation is the practice of applying less water than the ET of the crop or plant materials. Deficit irrigation is useful for conserving water in woody landscape ornamentals and drought tolerant plants where crop yield is not an issue. Water conserved in these areas may be re-allocated to other areas on the farm or landscape.
- There hasn’t been enough research on deficit irrigation of avocado for us to comment. We suspect, however, that deficit irrigation will simply lead to dropped fruit and reduced yield.
- Stumping the avocado tree could be considered a form of deficit irrigation. In this case, the tree should be stumped in the spring, painted with white water-based paint to reflect heat, and the sprinkler can be capped for at least 2 months. As the tree starts to re-grow, some water should be added back, probably about 10-20% of the normal water use of a mature tree.
- Regulated Deficit Irrigation for Citrus is an important method for saving water, and in some cases will reduce puff and crease of the peel. In one orange trial done by Dr. David Goldhammer in the San Joaquin Valley, an application of 25% of ETc from mid-May to Mid July saved about 25% of applied water for the year and reduced crease by 67%, without appreciably reducing yield.
- 3. Reduce Irrigated Area.
- Taking trees out of production. Trees that are chronically diseased and do not produce fruit (or the fruit is poor quality) should be taken out of production during this period. Also consider: trees in frosty areas, trees in wind-blown areas, trees near eucalyptus and other large trees that steal the water from the fruit trees.
- Changing crops. You may want to take out those Valencias during this period and replant to something that brings in more money, like seedless, easy-peeling mandarins. The young trees will be using a lot less water.
- Fallow Opportunities. You may decide to do some soil preparation, tillage or cultivation, or even soil solarization of non-irrigated areas.
We have found that this four step process is a logical way to achieve water cutbacks with least impact. It is possible to achieve a ten percent reduction in water by only improving irrigation system uniformity and scheduling procedures. Often, these two measures also result in better crop performance and reduced runoff. Reducing irrigated area or taking areas out of production should be a last resort and a well thought out decision. Plan for the future, hopefully water will be more available in future years.